WASHINGTON — President Obama is rolling out a campaign of private entreaties and public advocacy over the next several weeks to build support in Congress for the nuclear deal with Iran, an effort to counter a well-financed onslaught from critics who have promised to use a monthlong congressional recess to pressure lawmakers to oppose the accord.

In a speech at American University in Washington on Wednesday, Mr. Obama will seek to explain and defend the international agreement reached last month, which would lift some sanctions in exchange for restrictions on Iran’s ability to develop a nuclear weapon.

Mr. Obama, who campaigned for the presidency in 2008 promising to end wars in the Middle East, will use the speech to frame Congress’s choice as the most consequential foreign policy decision since the vote to go to war in Iraq, and he will say the deal’s opponents are the same people who supported that military conflict.

“He will make the case that this should not even be a close call,” a White House official said, speaking on the condition of anonymity to offer a preview of Mr. Obama’s message. The president will also say that it would be a “historic mistake to squander this opportunity — removing constraints on the Iranian program, unraveling the sanctions regime and damaging American credibility.”

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Prime Minister Benjamin Netanyahu of Israel spoke via webcast to Jewish institutions across the United States to outline his opposition to the Iran nuclear accord.Published OnAug. 4, 2015

Anticipating a month of heavy lobbying and television advertising by opponents, led by the pro-Israel group Aipac, the president and members of his team are leaning on Democrats to declare their backing for the agreement before they leave Washington to face their constituents.

Mr. Obama, who will decamp to Martha’s Vineyard this weekend for his own two-week vacation, will have limited personal contact with wavering lawmakers, but his team has been instructed to make the president and other senior administration officials available to any skeptic with an unanswered question or concern about the deal. “Anyone who wants a phone call will get one,” one official said, speaking on the condition of anonymity to outline internal strategy.

Officials said that Mr. Obama’s address on Wednesday would be followed by a series of news media interviews that would be shown next week. And the administration plans to dispatch cabinet members, including Energy Secretary Ernest J. Moniz, the nuclear physicist who helped negotiate the accord, to travel the country outlining its provisions. Mr. Moniz will appear on Friday at the Chicago Council on Global Affairs, officials said.

“We are confident that a sizable number of members of Congress will put politics aside and focus on what they believe is in the best interest of the United States and our national security, and if they do, a substantial number of those who follow that path will be supportive of the agreement,” said Josh Earnest, the White House press secretary. “There’s no denying that there is intense political pressure on both sides of this agreement,” he continued, adding that officials were hoping that lawmakers would “focus on the specific terms of the agreement.”

The effort gained some momentum Tuesday as three closely watched Democratic senators — Barbara Boxer of California, Tim Kaine of Virginia and Bill Nelson of Florida — declared their backing, along with a handful of House Democrats. Mr. Obama’s team is working with Representative Nancy Pelosi of California, the minority leader, to build support to potentially sustain a presidential veto of legislation rejecting the accord.

But some prominent voices, including Representative Steve Israel of New York, the highest-ranking Jewish Democrat in the House; Nita Lowey, also of New York; and Ted Deutch of Florida came out against the agreement.

“After a decade in public life working to stop Iran from ever acquiring nuclear weapons, I cannot support a deal giving Iran billions of dollars in sanctions relief — in return for letting it maintain an advanced nuclear program and the infrastructure of a threshold nuclear state,” Mr. Deutch, the senior Democrat on the House Foreign Affairs subcommittee on the Middle East, wrote in The Sun Sentinel.

Congress, which will return from its break Sept. 8, has until Sept. 17 to vote on a resolution supporting or rejecting the accord.

Public opinion on the agreement has been in flux in recent weeks, with some polls indicating majorities or pluralities supporting it and others showing more opposition. J Street, a pro-Israel group that supports the deal and has announced a $2 million advertising campaign, found that a majority of American Jews supported the agreement.

But Mark Mellman, a Democratic pollster who is a consultant to Citizens for a Nuclear Free Iran, an Aipac-funded group that is planning to spend more than $25 million on advertising in the coming weeks, said that the more people knew about the deal, “the more likely they are to disapprove of it.”

On Tuesday, Prime Minister Benjamin Netanyahu of Israel told thousands of American Jews in a webcast that the agreement was fatally flawed and dangerous, charging that proponents were trying to muzzle criticism of it with deceitful claims. “As a result of this deal, there will be more terrorism, there will be more attacks, and more people will die,” Mr. Netanyahu said.

He also angrily rejected Mr. Obama’s claims about the accord, particularly his argument that its opponents have no alternative other than war for reining in Iran’s nuclear weapons ambitions, calling it “utterly false.”

Mr. Netanyahu’s latest plea to American Jews came as Mr. Obama and Vice President Joseph R. Biden Jr. met privately at the White House with about 20 leaders of Jewish groups to try to allay concerns about the deal, the first such session since the pact was completed.

During a meeting in the Cabinet Room that lasted more than two hours, some of the leaders criticized Mr. Obama for portraying opponents of the deal “basically as warmongers,” one attendee said. The president said he would “be very careful about it,” but added that he would not hesitate to argue forcefully for the agreement, which he sees as being in the best interest of both the United States and Israel.

The White House also posted a copy of the 159-page agreement on the website Medium, annotated with comments from Secretary of State John Kerry, Mr. Moniz and Treasury Secretary Jacob J. Lew. “It’s important that everyone understands exactly what’s in this deal and how it’ll work,” the White House said in a preface to the post.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Obama Begins His Own Push for Iran Pact. Order Reprints | Today’s Paper | Subscribe